1. Field of the Invention
The present invention relates to the product and process for manufacturing moisture vapor impermeable cartons predominately from sheets of cellulosic fiber material that are suitable for packaging hygroscopic particulate material.
2. Description of the Prior Art
Due to an emerging public awareness of the cumulative deterimental impact on the environment of certain industrial and consumer waste compounds, the manufacture, shipment and marketing of some products and commodities has been exhaustively reviewed. Among such reviewed products are household detergents which historically have contained phosphate compounds to enhance the cleaning and dispersion properties thereof.
Since phosphate compounds originating from commercial detergent blends are thought to contribute significantly to the nutrient support of oxygen consuming organisms in natural streams and water bodies, powdered detergent manufacturers have sought more suitable, nutrient free alternatives for phosphates. Although many such alternatives have been found, nitrilotriacetic acid (NTA) for example, most if not all of such available alternatives are considerably more hygroscopic than the older phosphate compounds. Moreover, said alternatives are more susceptible to congealing and caking when subjected to water vapor. Accordingly, pressure has been brought to bear on the fiber carton suppliers to provide a more moisture vapor resistant package for the new, hygroscopic detergents.
Although there are many ostensibly suitable moisture-proof packaging materials and techniques available to detergent manufacturers, the criteria of cost, performance and attractiveness favor the selection of bleached paper board for the carton material. Accordingly, various laminated combinations of paper board, wax and/or thermoplastics have been proposed and used in the past.
Functionally, prior art systems of carton stock as represented by the U.S. Pat. Nos. 3,194,469 and 3,194,474 to George G. Rumberger have generally performed the intended purpose satisfactorily. It is the criteria of practicality and economics on which such prior art systems have fallen short of acceptability. Basically, prior art laminated carton stock is susceptible to severe warping and curling in the cross-machine direction (CD): i.e., the product web of uniform width and indefinite length curls about an axis parallel with the length thereof. The mechanisms of such warping and curling are present in the laminated composite as it emerges from the laminating machine and even though subsequently reeled and stored in a true cylindrical configuration with straight line surface elements, no correction of the CD curl is provided. Moreover, the undesirable curl condition is aggravated by passage through the multiple color stations of a rotogravure printing press.
Such curling and warping is believed to be caused by stress differentials between opposite face planes of the laminated sheet stock. When a vapor impermeable strata of thermoplastic separates face laminae of fiberboard, equalizing migration of moisture between the respective porous laminae is precluded. If one laminae is subject to more severe drying conditions than the other, an internal stress differential is thereby created and results in a bending or warping of the composite.
An example of such unequal drying conditions arises in a rotogravure printer where heat is applied to the printed face to drive out excess solvent deposited thereon as vehicle for the ink pigment. On the printed side of the vapor barrier, the fiberous laminae remains in moisture equilibrium. On the unprinted or liner side of the vapor barrier, subject to transversely conducted heating but without benefit solvent additions, a net drying occurs. Accordingly, a moisture content and consequent stress differential results.